There is a need for an accurate and responsive environmental sensor system that can locate the position and measure the amplitude of a time varying disturbance. There is also a need for strain sensors capable of measuring strain over both short and long gauge lengths. Applications of this type of sensor system would include the location and identification of vibrational disturbances in an aircraft or space structure as well as absolute strain measurements throughout the structure. These structures, in order to perform their mission, must be able to provide a continuous indication of their status in real time. Civil structure applications of this sensor would include the measurement of strain and acoustic disturbances throughout a bridge, building or highway structure. Agricultural applications would include the location of insect infestations in a grain storage area and localization of termite infestations in wooden housing structures. These sensors could also be used to support systems that could be used to identify structural damage to residential and business building after a disaster such as an earthquake or hurricane.
J. P. Dakin et. al. in "A Novel Distributed Optical Fibre Sensing System Enabling Location of Disturbances in a Sagnac Loop Interferometer", SPIE Proceedings, Volume 838, p. 325, 1987 describe a combination of a Sagnac and Mach-Zehnder interferometer that is used to locate the position of a time varying disturbance. There are a number of issues associated with this approach, the most serious of which is that optimum operation of the Sagnac and Mach-Zehnder interferometer in the configuration employed by Dakin et. al. requires two very different light sources. The Sagnac interferometer operates optimally with a low coherence length light source while the Mach-Zehnder would ideally be operated with a long coherence length, frequency stable light source. Further complicating matters is the Mach-Zehnder/Sagnac distributed sensor as described by Dakin results in light from the light source being directly reflected back into the light source, which without costly steps to provide suitable isolation, will result in a degradation of the light source stability and raising of the system noise floor. To circumvent this situation E. Udd in "Sagnac Distributed Sensor Concepts", Proceedings of SPIE, Volume 1586, p. 46, 1991, "Sagnac Distributed Sensor", U.S. Pat. No. 4,898,468, "Fiber Optic Detection System Using a Sagnac Interferometer", U.S. Pat. No. 5,046,848 and "Sagnac Based Secure Communication System", U.S. Pat. No. 5,311,592 describes a series of embodiments of distributed sensors based on the Sagnac interferometer that employ low coherence length light sources enabling optimum system performance.
One of these systems described by E. Udd in "Sagnac Distributed Sensor", U.S. Pat. No. 4,898,468 has the capability of making strain measurements over the entire Sagnac loop. To accomplish this, a frequency shifter that requires electrical power has been added to the Sagnac loop.
For many applications such as bridges, buildings, and aerospace platforms, it is highly desirable to have a system that is passive and has the ability to measure strain locally as well as over long gauge lengths while retaining the ability to locate and measure the amplitude of time varying effects.